1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, or a multi-functional apparatus having some combination of these functions, and a fixing unit disposed in the image forming apparatus, and more particularly, to an image forming apparatus having a fixing unit to increase and decrease a nip pressure between a fixing member and a pressure member.
2. Description of the Background Art
Conventionally, image forming apparatuses such as copiers and printers employ a fixing unit having a fixing member and a pressure member to conduct a fixing process, in which a fixing nip is set between the fixing member and the pressure member with a pressurized condition, and a recording medium is transported through the fixing nip for conducting a fixing process. For example, JP-2007-79160-A, JP-H10-282828-A, and JP-2006-23427-A disclose such configuration.
Furthermore, it is known that the size or extent of the nip width and the nip pressure at the fixing nip in the fixing unit greatly affects fixing performance. Further, because the recording medium receives relatively greater pressure and heat at the fixing nip position, after the recording medium exits the fixing nip, it is known that the recording medium sometimes experiences curling such as back curling. Specifically, the recording medium may be curved or warped along a curvature of the pressure member, and curved or warped to that side of the recording medium on which no image is formed (that is, the pressure member side). A technique is known for correcting such curling by, for example, correcting, canceling, or offsetting such curling, in which a second curling is applied to the recording medium in a direction opposite the direction of the first curling of the recording medium exiting the fixing nip.
Further, JP-2006-23427-A discloses another technology or method to correct curling of the recording medium involving a curling correction unit equipped with a curling guide. Depending on the thickness or other properties of the recording medium transported to the fixing nip, an angle of the de-curling guide is varied to provide effective curling correction.
In such conventional fixing units, depending on the type of recording media and an image area ratio of the output image, the pressure at a fixing nip (nip pressure) is adjusted so that a good fused image can be obtained consistently. However, the amount of curling of the recording medium may fluctuate because the nip pressure varies in response to the recording medium. If the fixing nip pressure is increased, the recording medium might curl more. In such a case, jamming of recording medium (or sheet jamming) may occur along a transportation route through which the recording medium is transported after the fixing nip, thus degrading image quality.